Java Developer interview questions typically cover core Java concepts, object-oriented programming, frameworks, and practical coding scenarios. Below are 25 commonly asked Java Developer interview questions with concise answers to help you prepare.

Q1. What are the main features of Java?

Java is platform-independent, object-oriented, secure, robust, multithreaded, and supports automatic memory management via garbage collection.

Q2. What is the difference between JDK, JRE, and JVM?

JVM runs Java bytecode, JRE provides runtime environment including JVM and libraries, JDK includes JRE plus development tools like compiler and debugger.

Q3. Explain the concept of Object-Oriented Programming (OOP) in Java.

OOP principles include encapsulation, inheritance, polymorphism, and abstraction to design modular and reusable code.

Q4. What is the difference between an interface and an abstract class?

Interfaces declare methods without implementation, support multiple inheritance. Abstract classes can have implemented methods and state, but Java supports single inheritance.

Q5. What is a constructor in Java?

A constructor is a special method used to initialize objects. It has the same name as the class and no return type.

Q6. What are Java access modifiers?

They define visibility: private, default (package-private), protected, and public.

Q7. What is the difference between equals() and == in Java?

equals() compares object content, while == compares references (memory addresses).

Q8. What are exceptions in Java? Name some common types.

Exceptions are runtime errors. Checked exceptions (e.g., IOException), unchecked exceptions (e.g., NullPointerException), and errors (e.g., OutOfMemoryError).

Q9. What is the purpose of the final keyword?

final can be applied to variables (constant value), methods (cannot be overridden), and classes (cannot be subclassed).

Q10. What is multithreading in Java?

Multithreading allows concurrent execution of two or more threads for better CPU utilization.

Q11. How do you create a thread in Java?

By extending Thread class or implementing Runnable interface and overriding run() method.

Q12. What is synchronization?

Synchronization controls access to shared resources in multithreaded environments to prevent race conditions.

Q13. What is a Java Collection Framework?

A set of classes and interfaces that implement commonly reusable data structures like List, Set, Map, and Queue.

Q14. Difference between ArrayList and LinkedList?

ArrayList uses dynamic array, offers fast random access; LinkedList uses doubly linked list, better for frequent insertions/deletions.

Q15. What is the difference between HashMap and Hashtable?

HashMap is non-synchronized and allows null keys/values; Hashtable is synchronized and does not allow null keys/values.

Q16. What is garbage collection in Java?

Automatic process of reclaiming memory by deleting unreachable objects to prevent memory leaks.

Q17. Explain Java memory model basics.

Java memory model includes heap (objects), stack (method calls and local variables), method area (class data), and program counter.

Q18. What is the difference between String, StringBuilder, and StringBuffer?

String is immutable, StringBuilder and StringBuffer are mutable; StringBuffer is synchronized while StringBuilder is not.

Q19. What are Java annotations?

Metadata that provides data about a program but is not part of the program itself, e.g., @Override, @Deprecated.

Q20. What is JDBC?

Java Database Connectivity is an API to connect and execute queries with databases.

Q21. What is the difference between checked and unchecked exceptions?

Checked exceptions must be handled or declared, unchecked exceptions (RuntimeExceptions) do not require explicit handling.

Q22. What is the significance of the main() method?

It is the entry point of any Java application and must be public static void main(String[] args).

Q23. Explain the concept of package in Java.

A package groups related classes and interfaces, helps avoid name conflicts and controls access.

Q24. How does Java achieve platform independence?

Java code compiles to bytecode which runs on JVMs specific to each platform.

Q25. What are lambda expressions in Java?

Introduced in Java 8, lambda expressions enable functional programming by allowing inline implementation of functional interfaces.